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Electrostatically Tunable Piezoelectric-onSilicon Micromechanical Resonator for Real-Time Clock Diego E. Serrano, Student Member, IEEE, Roozbeh Tabrizian, Student Member, IEEE, and Farrokh Ayazi, Senior Member, IEEE Abstract—This paper reports on the design, fabrication, and characterization of a small form factor, piezoelectrically transduced, tunable micromechanical resonator for real-time clock (RTC) applications (32.768 kHz). The device was designed to resonate in an out-of-plane flexural mode to simultaneously achieve low-frequency operation and reduced motional resistance in a small die area. Finite element simulations were extensively used to optimize the structure in terms of size, insertion loss, spurious-mode rejection, and frequency tuning. Microresonators with an overall die area of only 350 × 350 µm were implemented on a thin-film AlN on silicon-on-insulator (SOI) substrate with AlN thickness of 0.5 µm, device layer of 1.5 µm, and an electrostatic tuning gap size of 1 µm. A frequency tuning range of 3100 ppm was measured using dc voltages of less than 4 V. This range is sufficient to compensate for frequency variations of the microresonator across temperature from −20°C to 100°C. The device exhibits low motional impedance that is completely independent of the frequency tuning potential. Discrete electronics were used in conjunction with the resonator to implement an oscillator, verifying its functionality as a timing reference.
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are vital components in any electronic system that requires signal synchronization or time keeping. From military applications to consumer electronics, these frequency-selective devices play an important role in data processing and communications. For example, modern portable handsets rely on the use of stable timing references for the operation of their microprocessor (24MHz crystal oscillator; XO); the cellular module (26-MHz temperature-compensated crystal oscillator; TCXO); the GPS (33.6-MHz TCXO); and the sleep-mode, power management, and real-time clock (RTC) units (32.768kHz XO). In addition, other frequency components are required for narrowband and wideband filtering [1]. Because of their excellent, temperature-stable characteristics and their high quality factors, quartz crystals have been the choice of preference in high-accuracy timing applications [2]. Currently, commercially available resonators account for approximately 40% of the steadily growing $5 billion timing market [3]; hence, techniques to reduce size and cost, without a compromise in performance, are necessary to make new products competitive.
Manuscript received July 30, 2011; accepted November 11, 2011. This work was supported by Integrated Device Technology (IDT), Inc. The authors are with the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA (e-mail:
[email protected]). DOI: http://dx.doi.org/10.1109/TUFFC.2012.2204 0885–3010/$25.00
In the last few years, silicon micromechanical resonators have received much attention as a result of their small size, high quality factor (Q), and their potential for singlepackage or single-chip integration with their companion interface circuits [4], [5]. These devices have recently entered the market as strong contenders and now hold a significant share in applications operating in the megahertz range. On the other hand, very few efforts have been made in the development of alternative low-frequency (~32 kHz) resonators as replacements for bulky conventional tuning fork quartz crystals. Given that this type of structure requires precise fabrication steps to guarantee temperaturestable operation, and intricate packaging methods for vacuum encapsulation, they tend to be large in size (a few millimeters on a side) and impractical to integrate with CMOS circuitry. Therefore, silicon micro electromechanical systems (MEMS) resonators operating in flexural modes could serve as a viable solution to overcome these complications. In fact, some attempts to do so have been previously reported [6], [7]; however, these devices rely on electrostatic (i.e., capacitive) transduction, which suffers from low electromechanical coupling resulting in undesirably large motional impedances. To reduce these values, narrow capacitive gaps and large polarization voltages are required, compromising the device’s power handling and complicating its fabrication. Thin-film piezoelectric-on-silicon microresonators can be utilized to simultaneously enable piezoelectric transduction and electrostatic frequency tuning [8]. This approach results in lower motional impedances, because of the better electromechanical coupling provided by the piezoelectric effect, and the ability to adjust the operation frequency of the microresonator to compensate for process and temperature variations by the use of a dc potential applied between the device and the handle layers of a silicon-on-insulator (SOI) substrate. In this paper, a small form factor, voltage-tunable AlNon-SOI microresonator is presented as a miniature and cost-effective alternative to quartz crystals [9]. II. R D A. Mechanical Structure To achieve both low-frequency operation and reduced motional resistance in a small die area, the microresona-
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Fig. 1. Schematic representation of small form factor, piezoelectrically transduced, electrostatically tunable low-frequency resonator. Labels 1 through 4 correspond to each signal electrode. The device layer is in electrical contact with the ground electrode. The handle layer serves as a dc tuning electrode.
tor was designed to operate in an out-of-plane flexural mode using piezoelectric transduction. As shown in Fig. 1, the structure consists of an external frame composed of four clamped-clamped beams anchored at its corners and separated from the supporting substrate (i.e., handle layer of the SOI substrate) by a capacitive air-gap. The beams are mechanically coupled to each other through a suspended rigid plate that contributes additional mass to the system without increasing the structure stiffness. This, in turn, lowers the device’s resonance frequency. Furthermore, the plate provides a large surface area that couples electrostatically to the handle layer, serving as a frequency-tuning electrode. B. Piezoelectric Transduction The resonator is fabricated on an AlN-on-SOI substrate [10]. Molybdenum (Mo) layers are used above and below a thin-film AlN layer to enable electrical excitation and sensing of the resonance mode through the piezoelectric effect. Pairs of drive and sensor electrodes were defined above each tether by patterning the top Mo layer. The AlN layer was etched in certain areas to access the bottom Mo layer, which is in electrical contact with the single-crystal silicon (SCS) layer and serves as the ground electrode. By applying an ac voltage between the drive and ground electrodes, shear stress is generated over the beams through transverse piezoelectric coupling determined by the d31 coefficient of AlN. This stress produces a bending moment that excites the structure into its fundamental flexural resonance mode (Fig. 2). The displacement, amplified at resonance by the high quality factor of the device, induces a strain over the AlN layer on the beams at the opposite side of the structure. Because of an inverse piezoelectric effect, electric charge is excited on the sense electrodes, which can then be collected in the form of an ac current. Use of an out-of-plane mode instead of an in-plane mode is advantageous because the frequency value of a
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Fig. 2. ANSYS modal simulation results for fundamental out-of-plane flexural mode of the piezoelectric 32-kHz resonator.
flexural structure is highly dependent on the dimensions along the displacement axis. In the case of in-plane modes, these dimensions are determined by lithography, making the resonance frequency highly prone to process variations. On the other hand, in out-of-plane modes, the displacement occurs along the device thickness, which can be accurately controlled through material deposition steps, minimizing process dependencies. Furthermore, the use of a thin layer yields low device stiffness, which allows smaller lateral dimensions for the same frequency of operation. For instance, the devices presented herein occupy 1/3 to 1/5 the area of previously reported silicon resonators operating in in-plane modes at the same resonance frequency [6], [7]. In most electromechanical resonators, and mainly in piezoelectrically transduced devices, several modes can be excited into resonance using the same set of electrodes [11]. The unwanted modes can become problematic in the implementation of oscillators for timing applications, because closed-loop systems might lock into undesired resonance modes. Fig. 3 shows some of the spurious resonance modes that are close enough to the frequency of interest to be a potential problem. If proper design techniques are utilized, most of these modes can be suppressed or at least minimized. For instance, in the presented design, mode number 5 [Fig. 3(b)] is completely rejected, given that the beams connected to common electrodes vibrate in an outof-phase pattern, causing the changes in charge to cancel out. This effect can be verified with a harmonic simulation (Fig. 4) in which the absence of a peak response at the frequency of that particular mode confirms that the electrical inputs and outputs are decoupled. In reality, mismatches and process variations might cause the electrodes to be slightly different and some net charge could be picked up, but its contribution would still be negligible. It is also clear from Fig. 4 that, depending on the electrode configuration used to drive and sense the structure, certain modes can be completely suppressed. In configuration 1, electrodes 1 and 2 are used for excitation, whereas electrodes 3 and 4 are used for sensing. In this particular case, asymmetric rotational vibrations like the ones exhib-
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Fig. 3. ANSYS modal simulation results of potentially problematic spurious modes within 200 kHz of the fundamental frequency.
Fig. 4. ANSYS harmonic simulation results for different electrode configurations. In configuration 1, electrodes 1 and 2 serve as drive inputs, 3 and 4 serve as sensor outputs. In configuration 2, electrodes 1 and 3 serve as drive inputs, 2 and 4 serve as sensor outputs.
ited by modes 2 [Fig. 3(a)], 7 [Fig. 3(d)], and 10 [Fig. 3(f)], are vigorously excited because of the unbalanced nature of the electrode arrangement. On the other hand, if configuration 2 is selected, in which electrodes 1 and 3 are used as drive electrodes and 2 and 4 as sensor electrodes, only symmetric flexural vibrations of the beams can be excited, which is more desirable. Modes exhibiting the same vibration pattern over the beams as the fundamental mode [Figs. 3(c) and 3(e)] tend to be more difficult to suppress. However, because most of the mechanical coupling between the four tethers occurs through the out-of-plane displacement of the center plate, these modes display a higher insertion loss than the mode of interest. Fig. 4 shows that the fundamental mode is at least 10 dB higher than any other mode within 200 kHz, facilitating the implementation of a stable oscillator. C. Electrode Design In piezoelectric devices, the motional impedance is inversely proportional to the square of the electromechanical
coupling. To achieve low insertion loss values, the signal electrodes should be carefully sized and shaped to guarantee that the induced stress does not experience a change in sign at any point of the covered piezoelectric material area; this avoids charge cancellation and optimizes the coupling. It has been previously demonstrated that for piezoelectric-on-silicon clamped-clamped microbeams operating in their fundamental mode, the electrodes should cover up to 1/4th of the beam length to achieve maximum electromechanical transduction [12]. In the presented design, the displacement is determined only by the bending of the four tethers, so the same result is expected. Finite element simulations were used to sweep the electrode length and verify this effect. As shown in Fig. 5, the optimal admittance value is obtained when the electrode is 22.5% of the beam length rather than 25%. This discrepancy is due to the undercut of the device layer, which changes the effective beam anchor location and, hence, its maximum and minimum stress points. D. Electrostatic Tuning The main challenge posed by silicon-based resonators is their large frequency dependence on temperature. Because of their large temperature coefficient of elasticity (TCE), resonant structures implemented in native silicon exhibit a temperature coefficient of frequency (TCF) of about −30 ppm/°C. This value results in significantly larger frequency drifts across a wide temperature range compared with their quartz counterparts. For example, traditional X +5° cut quartz utilized for tuning fork implementations exhibits no more than 60 ppm of frequency variation from −20°C to 85°C. Therefore, to achieve the same or better level of performance, silicon microresonators must rely on additional temperature compensation techniques [13]–[17] or ovenization schemes [18], which can be implemented
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Fig. 5. Simulation results for electrode length optimization. The optimal simulated length is slightly smaller than the theoretical value because of undercut of the device layer.
either at the device level, within the interface circuitry [19], or both. Although the design reported herein does not require a polarization voltage to resonate, an electrostatic potential can be utilized as a frequency tuning mechanism to compensate for temperature and process variations. Given that the SCS resonator body is in electrical contact with the bottom Mo layer, its potential is always connected to ground. If a dc voltage Vtun is applied to the handle layer, an electrostatic force proportional to the device displacement is generated over the resonator. This force causes a spring softening effect that shifts the resonance frequency value. The relative change in frequency is given by 2 1 ε ⋅ Ae ⋅ V tun ∆f ≈− o , 3 2 km ⋅ g o fo
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Fig. 6. Theoretical and simulated (ANSYS) resonance frequency tuning characteristics for a 250 × 250 µm tunable piezoelectric resonator with capacitive gap of 1 µm.
Fig. 7. ANSYS static-structural/electrostatic finite element modeling simulation response for pull-in analysis. Snap-in of 32-kHz, 250 × 250 µm resonator occurs at 6.5 V.
(1)
where km is the mechanical spring constant, Ae is the effective capacitive area, and go is the initial capacitive gap [20]. Because the change in frequency is directly proportional to the effective area, the proposed design has significant advantages in terms of tuning compared with simpler structures with smaller areas, such as, for example, single clamped-clamped beam resonators. Fig. 6 shows the tuning curve for a device with plate area of 250 × 250 µm and a capacitive gap of 1 µm; it can be seen that a frequency tuning range of up to 6400 ppm can be achieved for a voltage of only 6 V. ANSYS (Ansys Inc., Canonsburg, PA) simulation results are in close agreement with the theoretical values. The maximum achievable tuning range is determined by the structure’s pull-in voltage, which for this particular design is close to 6.5 V. Results of a staticstructural/electrostatic finite element simulation confirm the pull-in behavior of the structure under an applied potential (Fig. 7). A larger frequency tuning range can be realized by adjusting the dimensions of the resonator and the capacitive gap-size. For instance, devices with plate areas of 500 × 500 µm and 1-µm gap provide frequency tuning of up to 15 000 ppm for voltages as low as 2 V, with a pull-in potential of 2.6 V.
Having separate mechanisms to excite and tune the structure is very advantageous because the tuning potential is not part of the signal path. This provides motional impedances that are independent of the tuning voltage, relaxing the requirements of the interface circuits required for the implementation of oscillators. Experimental results presented in Section IV confirm this behavior. III. R F Because of the low-frequency nature of the proposed microresonator, a thin device layer is required to achieve reduced structural stiffness, and therefore minimize the overall device area. This can be easily accomplished by the deposition or growth of materials whose thickness can be tightly controlled to make the resonator less susceptible to process variations. However, for compatibility purposes, the presented structure was implemented on a SOI substrate to facilitate the use of a previously established process flow, similar to the one developed for the fabrication of high-frequency thin-film piezo-on-substrate (TPoS) bulk-acoustic wave resonators [21]. The selected SOI wafers, which serve as the starting substrate, had been manufactured using separation
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Fig. 8. Fabrication process flow diagram of AlN on silicon-on-insulator resonator: (a) deposit Mo/AlN/Mo stack and pattern top electrode, (b) etch AlN to grant access to ground electrode, (c) pattern Si trenches and release holes to define geometry, (d) release device by etching buried oxide layer in HF.
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Fig. 10. Scanning electron micrograph close-up view of structure tether. A 1-µm capacitive gap remains between resonator and handle layer after etching the buried oxide layer.
Several resonators with different plate areas and tether widths were fabricated to evaluate the trade-offs between performance and size, and the reliability of the manufacturing. Fig. 9 shows an SEM view of a 32-kHz structure with a plate area of only 250 × 250 µm and 50-µm-wide tethers. A set of 8-µm-diameter release holes was introduced in the center plate to reduce the release time and thus decrease the undercut of the substrate. This allows the bonding electrodes to be brought closer to the device, shrinking the overall area. Fig. 10 shows a close-up view of one of the support tethers. It can be seen that the 1-µm buried oxide layer has been completely etched, leaving a capacitive gap between the released structure and the handle layer.
Fig. 9. Scanning electron micrograph top view of small form factor, 32kHz frequency-tunable piezoelectric microresonator.
by implantation of oxygen (SIMOX) to guarantee small variations in the thickness of the 1.5-µm silicon device layer and the 1-µm buried silicon dioxide layer. Low resistivity substrates (0.01 Ω·cm) were used to guarantee a uniform voltage distribution across the handle layer when the tuning potential is applied. A stack of Mo/AlN/ Mo was deposited on the substrate with thicknesses of 0.1/0.5/0.1 µm, respectively. The top Mo layer was patterned to define the top signal electrodes [Fig. 8(a)], followed by the etching of the AlN layer to provide access to the bottom ground electrode. AlN was also removed from the center portion of the beams and the rigid plate to reduce Q loading [Fig. 8(b)]. Lateral trenches and release holes were then patterned on the Si device layer to delimit the structure geometry [Fig. 8(c)]. Access to the handle layer to define tuning electrodes was also attained in this step. Finally, the device was released in hydrofluoric acid (HF), leaving a capacitive gap between the structure and the handle layer [Fig. 8(d)].
IV. M A. Resonator Characterization The microresonators were first tested in an open-loop configuration to characterize their resonance frequency, motional impedance, tuning range, and temperature behavior. The RF output of an Agilent AN4395A network analyzer (Agilent Technologies Inc., Santa Clara, CA) was used to excite the 32-kHz structure; electrode configuration 2 described in Section II was chosen because of its previously mentioned advantages. The device’s sensor electrodes were directly connected to the analyzer input channel in a high-impedance mode (1 MΩ termination) without any additional circuitry to amplify the signal. An Agilent E3631A dc power supply was connected to the handle layer to sweep the frequency tuning voltage. Fig. 11 shows the frequency response of the resonator for three different tuning potentials. It can be seen that, regardless of the applied dc voltage, both the motional impedance and the Q factor remain constant. This is a clear advantage over most capacitively-transduced resonators, in which the insertion loss is dependent upon the
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Fig. 11. Resonator response at frequency of interest. Insertion loss and quality factor are independent of the applied voltage.
Fig. 12. Resonator frequency tuning characteristics. The device achieves up to 3100 ppm of tuning for voltages no larger than 4 V.
polarization voltage, which is also used to tune the device frequency. To compensate for their intrinsic frequency dependency upon temperature, silicon resonators require large tuning capabilities to operate across a wide temperature range. Fig. 12 shows that by applying voltages no larger than 4 V, this particular design can achieve more than 3100 ppm of frequency tuning. Given that pull-in voltage of this structure is about 6.5 V, up to 6400 ppm can be achieved, if needed, by going up to 6 V. This extra margin can be used to compensate for any frequency deviation resulting from process variations. To characterize its frequency behavior across temperature, a resonator was placed in an environmental chamber in which the temperature was swept from −25°C to 100°C and back to −25°C. A TCF of −27.8 ppm/°C was measured at a constant dc potential; no hysteresis or nonlinear behavior was observed. The tuning voltage was then adjusted at each measured temperature point to bring the frequency back to its original value. TCF compensation was achieved for the specified temperature range. Fig. 13 shows the temperature characteristic of the device before and after calibration. No more than ±5 ppm of frequency drift was observed 5 h after the tuning voltage was adjusted. These small variations can be attributed to the temperature accuracy of the environmental chamber (±1°C).
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Fig. 13. Resonator temperature response before and after calibration. Tuning range is sufficient for temperature coefficient of frequency compensation from −25°C to 100°C.
Fig. 14. Oscillator implemented with 32-kHz microresonator and discrete electronics. An OPA656 and a feedback resistor of 470 kΩ constitute the trans-impedance amplifier (TIA). An LM311 and OPA227 (Texas Instruments, Dallas, TX) were used for the comparator and buffer, respectively. All filters were implemented with surface mount resistors and capacitors (Murata Manufacturing Co. Ltd., Nagaokakyo, Japan).
B. Oscillator Implementation To verify that the resonators are functional and can be used in a real-time system, a positive feedback loop was implemented in the form of an oscillator by means of discrete electronics (Fig. 14). The output current of the device’s sensor electrodes was amplified by a transimpedance amplifier (TIA) and then fed into a comparator. The amplitude of the voltage applied to the device’s drive electrodes was adjusted to guarantee that the resonator was not driven into the non-linear regime. High-pass and low-pass filters were used in the loop to reduce offset and avoid locking into high-order modes. All of the poles and zeros were selected to be at least 200 kHz away from the resonance frequency to prevent significant phase-shift contributions that could inhibit the system from locking into oscillations. The low-pass filter’s dominant pole was located at 450 kHz and the zero-pole pair of the high-pass filter at 0 and 34 Hz, respectively. These values introduce an overall phase delay of only 5° at the frequency of interest (32 kHz). Figs. 15(a) and 15(b) show the output voltage of the TIA and the input voltage to the resonator, respectively.
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Fig. 15. Measured results: (a) trans-impedance amplifier output voltage, and (b) input voltage to piezoelectric resonator.
The TIA signal exhibits an amplitude of 800 mVp-p at an oscillation frequency of 33.3 kHz, which corresponds to the fundamental mode value of the resonator used in this application. Given that the driving amplitude was set to 11 mVp-p and the TIA gain of 470 kΩ was selected, a motional impedance of about 6.5 kΩ is derived; this value is comparable to the resistances exhibited by high-accuracy tuning fork quartz crystals [22]. V. C This work demonstrates the implementation of a tunable piezoelectric MEMS resonator with a form factor of at least one order of magnitude smaller than current quartz crystals. The devices operate in an out-of-plane flexural mode to achieve low frequency operation and small die area. The devices were fabricated on an AlNon-SOI substrate to enable both piezoelectric excitation and electrostatic tuning. Extensive FEM modeling was performed to optimize the structure’s performance and die area. Measurement results show up to 3100 ppm of frequency tuning for TCF compensation in a temperature range of −25°C to 100°C utilizing no more than 4 V. The Q factor and motional impedance of the devices remain constant regardless of the tuning potential, facilitating the implementation of temperature-stable oscillators for realtime clock applications.
[1] C. S. Lam, “A review of the recent development of MEMS and crystal oscillators and their impacts on the frequency control products industry,” in IEEE Int. Ultrasonics Symp., 2008, pp. 694–704. [2] P. Rako, “Making oscillator selection crystal clear,” EDN, pp. 28–37, Feb. 19, 2009. [3] SiTime. (2011, May) SiTime enters $2 billion resonator market. [Online]. Available: http://www.sitime.com/news/239-sitimeenters-2-billion-resonator-market-with-worlds-first-mems-resonatorfor-real-time-clock-and-time-keeping-applications [4] F. Ayazi, “MEMS for integrated timing and spectral processing,” in Proc. IEEE Custom Integrated Circuits Conf., 2009, pp. 65–72. [5] C. T. C. Nguyen, “MEMS technology for timing and frequency control,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 54, no. 2, pp. 251–270, 2007. [6] K. R. Cioffi and H. Wan-Thai, “32kHz MEMS-based oscillator for low-power applications,” in Proc. IEEE Int. Frequency Control Symp. Exposition, 2005, pp. 551–558. [7] G. Ho and F. Ayazi, “Low frequency process-variation-insensitive temperature-stable micromechanical resonators,” U.S. Patent 7 859 365, Dec. 28, 2010. [8] G. Piazza, R. Abdolvand, G. Ho, and F. Ayazi, “Piezoelectricallytransduced, capacitively-tuned, high-Q single-crystal silicon micromechanical resonators on SOI wafers,” Sens. Actuators A, vol. 111, pp. 71–78, Mar. 2004. [9] D. E. Serrano, R. Tabrizian, and F. Ayazi, “Tunable piezoelectric MEMS resonators for real-time clock,” in Proc. Joint IEEE Int. Frequency Control Symp. European Frequency and Time Forum, 2011, pp. 765–768. [10] G. K. Ho, R. Abdolvand, A. Sivapurapu, S. Humad, and F. Ayazi, “Piezoelectric-on-silicon lateral bulk acoustic wave micromechanical resonators,” J. Microelectromech. Syst., vol. 17, no. 2, pp. 512–520, 2008. [11] A. K. Samarao and F. Ayazi, “Combined capacitive and piezoelectric transduction for high performance silicon microresonators,” in IEEE Int. Conf. Micro Electro Mechanical Systems, 2011, pp. 169– 172. [12] D. L. DeVoe, “Piezoelectric thin film micromechanical beam resonators,” Sens. Actuators A, vol. 88, no. 3, pp. 263–272, 2001. [13] R. Melamud, S. A. Chandorkar, K. Bongsang, L. Hyung Kyu, J. C. Salvia, G. Bahl, M. A. Hopcroft, and T. W. Kenny, “Temperatureinsensitive composite micromechanical resonators,” J. Microelectromech. Syst., vol. 18, no. 6, pp. 1409–1419, 2009. [14] S. Pourkamali, A. Hashimura, R. Abdolvand, G. K. Ho, A. Erbil, and F. Ayazi, “High-Q single crystal silicon HARPSS capacitive beam resonators with self-aligned sub-100-nm transduction gaps,” J. Microelectromech. Syst., vol. 12, no. 4, pp. 487–496, 2003. [15] A. K. Samarao, G. Casinovi, and F. Ayazi, “Passive TCF compensation in high Q silicon micromechanical resonators,” in IEEE Int. Conf. Micro Electro Mechanical Systems, 2010, pp. 116–119. [16] R. Tabrizian, G. Casinovi, and F. Ayazi, “Temperature-stable highQ AlN-on-silicon resonators with embedded array of oxide pillars,” in Solid-State Sensors, Actuators, and Microsystems Workshop, 2010, pp. 100–101. [17] H. Wan-Thai and C. T. C. Nguyen, “Stiffness-compensated temperature-insensitive micromechanical resonators,” in IEEE Int. Conf. Micro Electro Mechanical Systems, 2002, pp. 731–734. [18] S. H. Lee, J. Cho, S. W. Lee, M. F. Zaman, F. Ayazi, and K. Najafi, “A low-power oven-controlled vacuum package technology for highperformance MEMS,” in IEEE Int. Conf. Micro Electro Mechanical Systems, 2009, pp. 753–756. [19] D. Ruffieux, F. Krummenacher, A. Pezous, and G. Spinola-Durante, “Silicon resonator based 3.2 uW real time clock with +/−10 ppm frequency accuracy,” IEEE J. Solid-State Circuits, vol. 45, no. 1, pp. 224–234, 2010. [20] K. Sundaresan, G. Ho, S. Pourkamali, and F. Ayazi, “Electronically temperature compensated silicon bulk acoustic resonator reference oscillators,” IEEE J. Solid-State Circuits, vol. 42, no. 6, pp. 1425– 1434, 2007. [21] W. Pan and F. Ayazi, “Thin-film piezoelectric-on-substrate resonators with Q enhancement and TCF reduction,” in IEEE Int. Conf. Micro Electro Mechanical Systems, 2010, pp. 104–107. [22] E. Momosaki, “A brief review of progress in quartz tuning fork resonators,” in Proc. IEEE Int. Frequency Control Symp., 1997, pp. 552–565.
.: Diego E. Serrano (S’08) received the B.S. degree in electronics engineering from the Pontificia Universidad Javeriana, Bogotá, Colombia, in 2007 and the M.S. degree in electrical engineering from the Georgia Institute of Technology, Atlanta, GA, in 2009, where he is currently pursuing the Ph.D. degree. His research interests are in the areas of micromechanical sensor and resonator design and interface IC design for MEMS. Mr. Serrano was the recipient of the 2011 Goizueta Foundation Fellowship, an award granted to outstanding Hispanic graduate students at the Georgia Institute of Technology.
Roozbeh Tabrizian (S’06) received the B.S. degree in electrical engineering from Sharif University of Technology, Tehran, Iran, in 2007. He then joined the Georgia Institute of Technology, Atlanta, GA, in 2008, where he is currently working toward the Ph.D. degree, with a focus on the design, fabrication, and characterization of piezocapacitive micromechanical resonators and filters for timing and signal processing applications. Mr. Tabrizian was the recipient of the Outstanding Paper Award at the 15th International Conference on Solid-State Sensors, Actuators, and Microsystems (Transducers 2011).
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Farrokh Ayazi (S’96–M’00–SM’05) received the B.S. degree in electrical engineering from the University of Tehran, Iran, in 1994 and the M.S. and Ph.D. degrees in electrical engineering from the University of Michigan, Ann Arbor, in 1997 and 2000, respectively. He joined the faculty of the Georgia Institute of Technology, Atlanta, GA, in December 1999, where he is currently a professor in the School of Electrical and Computer Engineering. His research interests are in the areas of integrated micro- and nano-electromechanical resonators, interface IC design for MEMS and sensors, inertial sensors, RF MEMS, and microfabrication techniques. Prof. Ayazi is a 2004 recipient of the NSF Career Award, the 2004 Richard M. Bass Outstanding Teacher Award (determined by the vote of the ECE senior class), and the Georgia Tech College of Engineering Cutting Edge Research Award for 2001–2002. He is an editor for the IEEE/ASME Journal of Microelectromechanical Systems. He served on the technical program committee of the IEEE International Solid State Circuits Conference (ISSCC) for six years (2004 to 2009). He and his students won the best paper awards at Transducers 2011, the IEEE International Frequency Control Symposium in 2010, and IEEE Sensors conference in 2007. Dr. Ayazi is the co-founder and Chief Technology Officer (CTO) of Qualtré Inc., a spin-out from his research Laboratory that commercializes multi-axis BAW silicon gyroscopes and multi-degree-of-freedom inertial sensors for consumer electronics and personal navigation systems.
